Title: Limits on chemical and convective isolation in the Earth's interior
Abstract: Element transport between the mantle and the continents has been investigated for the SmNd, RbSr, ThUPb and LuHf radiogenic isotope systems using simultaneous numerical solutions to linear transport equations. Models incorporate up to three reservoirs: continental crust plus lithospheric mantle and two other mantle regions. Different combinations of inter-reservoir mass fluxes, including both uni- and bi-directional transport, are considered. Models are evaluated according to how well they satisfy compositional and evolutionary criteria of the Earth: (1) final concentrations and isotope ratios in the continents plus lithospheric mantle and the depleted mantle; (2) positive ϵNd(T) in the depleted mantle from 3.8 Ga B.P. onwards and approximately linear evolution; (3) Th/U of ∼2.5 in the depleted mantle today and time-integrated Th/U of ∼3.75, derived from Pb isotopes; (4) mean isotopic ages of 2.0–2.5 Ga for Nd in the continents; and (5) a nearly uniform lithosphere composition through time. The amount of mantle processed in forming the continents is derived by elemental and isotopic mass balance. Results are consistent with previous suggestions calling for a depleted region comprising 0.3–0.5 of the mantle, implying that portions of the mantle exist that are chemically and thus convectively distinct from one another. The deduced present-day chemical and isotopic compositions for the SmNd and RbSr systems in the upper mantle and the crust plus lithospheric mantle are reproduced in all the models considered. Consequently, the present state of RbSr and SmNd isotopes provides little clue as to which of the possible transport processes have produced them. The evolution of these systems, however, differs considerably between the models, as do the present-day ThUPb systematics, and provides a basis for discriminating between them. Models in which the continents grow by uni-directional transport from a depleting mantle and those in which the depleted mantle mass diminishes with time do not satisfy criteria (2) and (3) above simultaneously, even with the imposition of time-dependent fluxes. However, all of the criteria are met by models in which depleted mantle abundances of highly incompatible elements are maintained at near steady state by influxes from other reservoirs. If recycling of continent occurs, then it must occur in bulk; pelagic sediment subduction as the principal means of recycling appears to be ruled out. The alternative to bulk continent recycling is an influx into the depleted upper mantle of mantle material which is less depleted. This may involve an upper mantle of fixed mass exchanging at its base, or alternatively one growing through time. These are consistent with mantle convection influenced by a phase change, chemical boundary, or viscosity contrast, such that trace-element chemistry varies significantly between the shallow and the deeper mantle. An important feature common to the successful models is that the concentrations of highly incompatible elements in the reservoir representing the upper mantle are maintained by influxes from a less depleted region of the Earth, and that their residence times in the upper mantle are much less than the mean age of the continents.
Publication Year: 1989
Publication Date: 1989-06-01
Language: en
Type: article
Indexed In: ['crossref']
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Cited By Count: 98
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